SUBSURFACE SEQUENCE DELINEATION AND SALINE WATER MAPPING OF LAGOS STATE, SOUTHWESTERN NIGERIA

A subsurface sequence delineation and saline water mapping of Lagos State was carried out. Ten (10) deep boreholes with average depth of 300 m were drilled within the sedimentary basin. The boreholes were lithologically and geophysically logged. The driller's lithological logs aided by gamma and resistivity logs, showed that the area is underlain by clays, sandy/silty clays, clayey sands and sands with shale and limestone in places. The resistivity logs delineate saline water at shallow depth at Lekki wells I and II, Lakowe and Akodo with fresh/saline water interface at depths ranging from 8 - 15m. Confined saline water was delineated at deep levels at Eredo and Akodo. Saline seawater incursion has extended in land to within 2.5 - 12.5km from the coastline. KEY WORDS: Subsurface Sequence, Saline Water Mapping Global Jnl Geological Sciences. Vol.2(1) 2004: 111-12

Laboratory modelling of geoelectric response of a leaking underground petroleum storage tank in sand formation

Laboratory modelling of a simulated subsurface hydrocarbon impacted sand was carried out using a model tank. A crude oil filled metallic cylindrical tank (simulating a typical crude / refined hydrocarbon storage tank) was buried in the sand and pre-impact resistivity measurements with various electrode arrays (Wenner, dipole-dipole and gradient) were carried out along seven traverses, three of which traversed the buried cylindrical tank. A subsurface impact via crude oil leakages from the sides of the cylinder was initiated and post-impact resistivity measurements were made with same electrode arrays and along the same traverses. The study was aimed at generating characteristic geoelectric signatures or patterns for the pre-impact and post-impact hydrocarbon-impacted sand. Results from the resistivity profiles, maps and pseudosections as well as the inversion of the dipole-dipole data indicate diagnostic relatively high resistivity anomalies over the hydrocarbon-impacted zone. Although the three electrode arrays resolved the lateral extent of the impacted zone, a combined horizontal/vertical profiling involving dipole-dipole array proved more useful in determining the lateral and vertical extent, i.e., the geometry of the contaminated zone. It also enabled an approximate estimation of the volume of leaked hydrocarbon. Apart from resolving the hydrocarbon-impacted zone, the method also identified the source of impact. KEY WORDS: subsurface, leakage, resistivity, anomaly. Global Journal of Geological Sciences Vol.2(2) 2004: 207-22

Integrated VLF - Electromagnetic And Electrical Resistivity Survey For Groundwater In A Crystalline Basement Complex Terrain Of Southwest Nigeria

Very Low Frequency Electromagnetic (VLF-EM) and Electrical Resistivity (ER) methods were integrated in a feasibility study of a basement complex area for groundwater development. Linear features, suspected to be basement fractures, inferred from the VLF - EM anomaly curves, were confirmed by subsurface geoelectric images developed from interpretation results of the Vertical Electrical Soundings (VES). The delineated basement fractures were penetrated by three test boreholes at various depth ranges of between 7.0 and 13.0 m beneath Borehole (BH) 3 & 4 and 9.0 - 42 m beneath BH5 for unconfined fractures and 36.0 - 38.0 m beneath BH3 & 4 and 128.0 - 188.0 m beneath BH5 for confined fractures. Borehole yields vary from 0.33 - 2.0 l/s, where two abortive (dry) boreholes (BH1 & 2) were previously drilled. KEY WORDS: VLF - EM, Electrical Resistivity, Linear Features, Groundwater Development, Basement Complex Area. Global Journal of Geological Sciences Vol.3(1) 2005: 71-8

Geophysical and hydro-chemical investigation of the area around a waste dump site in Ile-Ife, Southwest Nigeria

No Abstract

Geoelectric characterization of Aquifer types in the basement complex terrain of parts of Osun state,Nigeria

No Abstract. Global Journal of Pure and Applied Sciences Vol. 12(3) 2006: 377-38

Computer model study of the effect of resistive topsoil on interpretation results of subsurface geoelectric layers' parameters

No Abstract

Assessment of anomalous seepage conditions in the Opa dam embankment, Ile-Ife, Southwestern Nigeria

An integrated geophysical survey, involving the vertical electrical sounding (VES), dipole-dipole electrical horizontal profiling, spontaneous potential (SP) profiling and magnetic profiling, was carried out along the embankment of the Opa dam located within the campus of the Obafemi Awolowo University, Ile-Ife, Nigeria. The aim of the investigation was to map possible anomalous seepage zones along the embankment of the dam. The geoelectric section revealed three distinct layers, namely the clayey sand/lateritic (cap rock) topsoil, the clay/sandy clay weathered layer (core) and the basement (bedrock). The dipole-dipole delineated anomalously low resistivity zones suspected to be seepage zones within stations 8 and 15 (40-75 m), 19-25 (95-135 m), 26-32 (130-160 m) and 38-42 (190-210 m) from the northwestern end of the embankment. SP total field and gradient profiles identified two major interfaces within the anomalously low resistivity zones at around stations 14 (70 m) and 20 (100 m). These are the major seepage zones. Verdant vegetation on the downstream side of the embankment and location of water ponds at the toe of the embankment around the identified seepage zones are field confirmation of the anomalous seepage. KEY WORDS: Anomalous seepage; Assessment; Opa Dam Embankment. Global Journal of Geological Sciences Vol.2(2) 2004: 191-19

ON THE APPLICATION OF THE ELECTRICAL RESISTIVITY METHOD IN FOUNDATION FAILURE INVESTIGATION – A case study

An investigation of the cause(s) of the foundation failure of the buildings in the premises of the Dental Clinic in the campus of the Obafemi Awolowo University (O.A.U), Ile-Ife, Nigeria, was carried out using the electrical resistivity method. The aim of the investigation was to delineate the geoelectric and geologic parameters of the subsurface, as a means of determining the cause(s) of the foundation failure. Measurements involving vertical electrical sounding (VES) and horizontal profiling (EHP) techniques were taken along three traverses. The results were presented as geoelectric sections, pseudosections and maps. The geoelectric sections reveal three subsurface layers, namely the topsoil, the weathered layer and the basement bedrock. Depth to the bedrock map revealed a depression with an approximate N-S trend. The competent materials underlie the premises. It is concluded from these that the buildings failed due to the flow of the incompetent clays on which they were founded. KEY WORDS: Electrical Resistivity, Foundation Failure, Case Study. Global Jnl Geological Sciences. Vol.2(1) 2004: 139-15